When writing temporary plugin files, ensure the directory exists

When adding plugins as streams in PluginIT (which upcoming commits
will do), installing the plugin failed, as the temporary directory
does not exist.

Instead of fixing the caller of `asTemp` to create the directory
before the call, we instead change `asTemp` itself to create the
directory, and can thereby simplify other callers, which also
manually create the directory.

Change-Id: If7962ee2898f52c0db43cf030528a82530a2442b

java/com/google/gerrit/server/plugins/JarPluginProvider.java

@@ -110,9 +110,6 @@ public class JarPluginProvider implements ServerPluginProvider {
 
   public static Path storeInTemp(String pluginName, InputStream in, SitePaths sitePaths)
       throws IOException {
-    if (!Files.exists(sitePaths.tmp_dir)) {
-      Files.createDirectories(sitePaths.tmp_dir);
-    }
     return PluginUtil.asTemp(in, tempNameFor(pluginName), ".jar", sitePaths.tmp_dir);
   }
 

java/com/google/gerrit/server/plugins/PluginUtil.java

@@ -53,6 +53,7 @@ public class PluginUtil {
   }
 
   static Path asTemp(InputStream in, String prefix, String suffix, Path dir) throws IOException {
+    Files.createDirectories(dir);
     Path tmp = Files.createTempFile(dir, prefix, suffix);
     boolean keep = false;
     try (OutputStream out = Files.newOutputStream(tmp)) {